This invention relates to an optical linear encoder used for a position measurement in a machine tool and the like and, in particular, to a position detector using moire fringes produced through a set of diffraction gratings.
Nowadays, a length or distance measurement method using moire fringes produced by a pair of diffraction gratings has been used widely, since moire fringes are sensitive to any change of lateral movement and are capable of measuring displacements by fine steps. A pair of diffraction gratings (hereinafter referred to as a first grating and a second grating) are attached to two members or parts of the machine tool, which parts move relative to each other, so that it has been necessary to always keep a distance or gap between the pair of diffraction gratings. While, as the pitch of each of the diffraction gratings is made small in order to improve the resolution or resolving power of the length measurement, the influence of the of light diffraction effect becomes large. Consequently, a shadow of the first grating on the second grating is made thin and it is therefore impossible to obtain direct moire fringes with a high visibility. In order to solve the shortcoming of the prior art, Fourier images have been used to obtain diffraction moire fringes. Fourier images refer to a distribution of dark and bright portions due to light shading. When the first grating has parallel light beams having uniform phase or coherence impinging thereon, a light shading or distribution having the same pitch as that of the first grating is obtained at the positions determined by intergral-number-multiplying the distance which is fixed by dividing twice the grating pitch P squared by a divisor equal to a wavelength of the light beams. (At the mid-positions of the positions noted above, that is, positions determined by multiplying by an integer divided by two the distance noted above, a light shading having a reversed relationship of dark and bright portions is attained.)
When the second grating is placed at the position where the Fourier image is produced and the two gratings move laterally and relatively, the diffracted light passing through the second grating shows a clear contrast of a period P, which is called a diffraction moire pattern. Recently this principle has been considered for use in short distance measurement, such as in a mask-alignment step of a fine semiconductor manufacturing process (see, for example, J. VAC. SCI. TECHNOL. 15 (1987), pp 984 and J. V. VAC. SCI. TECHNOL. B1 (1983), pp 1276).
When the distance to be measured is made long and the grating pitch P made short in order to raise the accuracy in a distance measurement, it is difficult to surely hold two diffraction gratings having longitudinal separated positions of a distance 2P.sup.2 /.lambda. enabling the production of a Fourier image, since the distance is abruptly shortened in proportion to the squared grating pitch P. When the distance or gap between the diffraction gratings changes or shifts from that enabling the production of a Fourier image, the intensity of diffracted light greatly changes, resulting in an impossibility of the positioning of the members of a machine tool. For example, presuming a diffraction grating pitch P is 1 .mu.m and a light beam having a wavelength .lambda. of 0.633 .mu.m is used, a change of the distance or gap G of the diffraction grating must be restricted within a sufficiently small range relative to 1.6 .mu.m giving a Fresnel number (.lambda..multidot.G)/P.sup.2 =2, which is obtained by dividing the product of the gap G of the diffraction gratings and the light wavelength .lambda. by a divisor equal to the diffraction grating's pitch P squared. That is the reason why diffraction moire fringes can not be used to finely measure or determine any distance between two members of generally used machine tools.
Under the circumstances, Japanese Laid-Open Patent Application No. 61-17016 of the applicant of the present application discloses a position detector for precisely determining any position, which determination is attained by a diffraction moire pattern not influenced by a change of the gap between the first grating and the second grating and sensitive to lateral displacement of these gratings.
According to the prior art, at every portion of the effective confrontation area between the first grating and the second grating, the light path or passage distance of the gap formed between the gratings changes to obtain a signal using a photodetector corresponding to the mean value of the diffracted moire signals. The correct position is detected by using the change in the signals of a period corresponding to that half the pitch P of the diffraction gratings, which change is seen in the mean value.
FIG. 1 to FIG. 3, respectively shows each example of the averaged diffraction moire position detector mentioned above. The operation of the position detector when a zero-order diffraction laser beam is used therein will be described.
In the mechanism shown in FIG. 1, the first grating 1 has a laser beam LB shown on the left impinging thereon. It is noted that the second grating 2 placed at the rear of the first grating 1 has a stepped transparent plate 3 attached to the second grating 2. The stepped transparent plate 3 is made of a high refractive index material selected so that the optical range of the gap G is between G.sub.o and G.sub.o +2P.sup.2 /.lambda.. The stepped transparent plate 3 gives light path distance differences to each part of the laser beam LB. The stepped transparent plate 3 of FIG. 1 has five steps, thereby dividing the range of the optical distance 2P.sup.2 /.lambda. into five parts. The lens group 4 serially arranged at a position behind the second grating 2 converges respective laser beams passing through respective five regions of the second grating 2, which regions have different optical distances.
Respective laser beams converged by each lens of the lens group 4 are respectively detected by means of a set of photodetectors 5. An adder 7 constructed of an operational amplifier and its related parts adds the signals from the photodetectors 5 to obtain displacement signals.
In the case shown in FIG. 2, the first grating 1 and the second grating 2 are placed in parallel, the later having a random light path difference plate 9 attached thereto or formed integrally therewith. The random light path difference plate 9 is made of a transparent material having a concavo-convex surface. The concavo-convex surface randomly determines the different light path distances of each part of the laser beam LB within the range of 2P.sup.2 /.lambda.. Respective parts of the laser beam LB converge to the diffusion plate 10 through the lens group 4. The focus points of the laser beam passing through each lens of the lens group 4 are placed on the diffusion plate 10 in a vertical line. That is, each part of light beam focussed or converged becomes incoherent by the diffusion plate 10. Laser beams diffused by the diffusion plate 10 pass through a convex lens 11 and a photodetector 12 such as a photodiode and the like detects the laser beam as shown in FIG. 2. Because the diffusion plate 10 is used and respective laser beams travel through different gap distances or different light path distances, these laser beams are averaged without mutual interference.
FIG. 3 shows another conventional case, in which the first grating 1 is placed vertically with respect to the incident direction of the laser beam LB and the second grating 2 is slanted with respect to the first grating 1. The gap distance of the effective confronting area between the gratings 1 and 2 is controlled so as to contain the range of 2P.sup.2 /.lambda.. Consequently, only zero-order diffracted light or the beam L.sub.o of the laser beam passing through the first and the second diffraction gratings 1 and 2 strikes a light receiving face of a photodetector 13 and is detected.
FIG. 4 is a perspective view similar to that of FIG. 3 of an example of an averaged diffraction moire position detector using second order diffracted light, in which the gap between the gratings 1 and 2 is controlled so as to contain the distances obtained by multiplying P.sup.2 /4.lambda.. If the second order diffracted light is used, the shading produced when zero-order diffracted light is used is formed at a position determined by multiplying P.sup.2 /4.lambda., and this second order diffracted light position differs from that when zero-order diffracted light is used. It is suitable to average the gap light path distances which are obtained by dividing the other gap light path distance of 2P.sup.2 /.lambda. to be averaged when zero-order diffracted light is used by eight. By the way, if second order diffracted light is used, even through it is controlled to contain the range of 2P.sup.2 /.lambda. which is identical with that when zero-order diffracted light is used, the condition of the averaged gap light path distance obtained when second order diffracted light is used is attained, since the distance corresponds to P.sup.2 /4.lambda. multiplied by a whole number. It is understood that if a second order diffracted light (included other number-order diffracted ones) is used in a similar optical system, it is possible to precisely detect the position without any influence of change in the gap between the first and the second gratings, to the result of measurement, similarly to the case when zero-order diffracted light is used.
According to the various conventional averaged diffraction moire position detectors mentioned above, a light intensity I changes according to a relative displacement X of respective gratings as shown in FIG. 5 and it is possible to obtain a displacement signal having a period which is half the pitch P of the gratings without any influence of change in the gap distance between the first and the second gratings to the measurement result. The displacement signal can be approximately expressed by the following equation (1). EQU I(x)=A.multidot. cos (2.pi..multidot.2x/P)+B (1)
wherein
A: amplitude PA1 B: offset component
However, if any difference occurs between the gap light path distances to be averaged while an actual assembling procedure and an operation thereof, and another light path distance actually averaged, or if some installation conditions have any error, the displacement signal obtained may contains some error components having a period of the pitch P of the grating and/or another error components of an odd number-order. Disadvantageously, when the displacement signal contains such error components, it becomes impossible to carry out a precise position detection, since repeatability or reproduceability of the displacement signal of a period P/2 can not be obtained.